A breath or blood test in a DUI case is always taken after the driving, sometimes long after. Montana law allows that. The statute asks for a test taken "within a reasonable time," and the Montana Supreme Court has held that the State does not have to calculate backward to the moment of driving. The gap still matters as a question of evidence. This page is part of our Montana DUI Guide.

Why the Timing Matters

Alcohol concentration is not constant. NHTSA's public guide, The ABCs of BAC, says alcohol is absorbed through the stomach and small intestine and can be measured within 30 to 70 minutes after a drink, and that food slows absorption. After the peak, the level falls as the body eliminates the alcohol.

So a person who finished a drink shortly before driving may still be absorbing alcohol when stopped. Their level at the station could be higher than it was at the wheel. A person stopped hours after drinking is likely on the way down, and the later test understates the earlier level. Which situation applies depends on facts the test alone does not reveal.

What the Statute Says About When the Test Is Taken

Montana's DUI statute addresses timing in one place. In a case alleging driving under the influence of alcohol, the inferences drawn from a test apply to a sample "drawn or taken within a reasonable time after the alleged act" (MCA § 61-8-1002(2)). There is no hour count in the statute.

The 0.08 offense itself is worded differently. It prohibits driving "while the person's alcohol concentration, as shown by analysis of the person's blood, breath, oral fluid, or other bodily substance, is 0.08 or more" (MCA § 61-8-1002(1)(b)). Read literally, that points to the moment of driving. That reading was the argument in McGowan.

State v. McGowan: No Extrapolation Required

Dennis McGowan was stopped in Helena for speeding. About 50 minutes later, a breath test at the detention center showed 0.092. He argued the State had not proved he was at 0.08 while driving, because alcohol levels keep rising for a time after drinking stops and his could have been lower on the road.

The Montana Supreme Court affirmed the conviction (State v. McGowan, 2006 MT 163, 332 Mont. 490, 139 P.3d 841). It read the DUI statutes together and held that breath tests "administered within a reasonable amount of time after the alleged act" are consistent with the per se statute (¶ 17). It declined to require retrograde extrapolation, noting that such an estimate depends on information usually known only to the defendant, such as when and how much the person drank, and that absorption rates vary widely between people (¶¶ 18 to 21).

McGowanwas decided under the statute numbers in use before 2022. The current statute uses substantially the same wording, and the Department of Justice's 2023 reference manual for prosecutors says earlier case law is generally assumed to continue to apply. The decision was not unanimous.

State v. Hala: How Long Is Reasonable?

In State v. Hala, 2015 MT 300, 381 Mont. 278, 358 P.3d 917, a driver crashed around 12:30 a.m. on a rural highway and was not found until about 5:00 a.m. At a trooper's request, blood was drawn at the hospital at about 8:30 a.m., roughly eight hours after the crash. The Court held the test was taken within a reasonable time, because the officer "did not unreasonably delay initiation of the test, given the circumstances of the accident" (¶ 14).

Hala describes the rule as a look at the totality of the circumstances (¶ 12). The practical lesson runs both ways. A long gap explained by the facts will be accepted. A gap caused by unexplained delay is open to challenge.

Retrograde Extrapolation in Practice

Retrograde extrapolation means working backward from a later test to an earlier level using an assumed elimination rate. It requires assumptions about when the person stopped drinking, whether they had finished absorbing the alcohol, and how fast their body clears it.

In two reported Montana cases the State's witness did not attempt it. In State v. Larson, 2004 MT 345, a State Crime Lab witness testified that assigning a specific earlier number was against the lab's policy at that time. The trial court excluded extrapolation and allowed him to say only that the driver would have been in the elimination phase when the blood was drawn, and the Supreme Court affirmed. In Hala, the State's witness likewise did not calculate a precise earlier level. When either side does offer an extrapolation, the assumptions behind it are the place to look.

When a Rising-Level Argument Has Force

  • The result is close to a legal line. A 0.09 leaves room for the question. A 0.20 does not.
  • The last drink was shortly before the stop. Receipts, card records, witnesses, and video can establish the timeline.
  • The test came well after the driving. The breath ticket or lab form shows the time.
  • The other evidence is thin. Steady driving and good balance on video are consistent with a lower level at the wheel.

The argument has limits. It does not help with an impairment chargesupported by strong observations. And it works against a driver whose level was probably falling. In an impairment case, a result of 0.08 or more allows an inference that the person was under the influence, but "the inference is rebuttable" (MCA § 61-8-1002(2)(c)), and timing evidence is one way to rebut it.

Records That Establish the Timeline

The stop time is in the dispatch log and on the video. The test time is printed on the breath record or the lab request. On request, full information about any test given at an officer's direction must be made available to you or your attorney (MCA § 61-8-1019(3)). You may also ask for an independent blood test at your own expense. For how a number is produced, see how BAC is measured, and for the other ways a result is tested, see challenging the breath test.

This page explains Montana law in general as of the 2025 Montana Code Annotated and the court decisions cited. Every case turns on its own facts. It is not legal advice about your specific case.